This work will lay the foundation for a long-term project to understand, at a molecular level, the mobility of the SCCmec element that renders Staphylococcus aureus resistant to a broad variety of penicillins. The resulting MRSA (methicillin resistant S. aureus) strains have become a serious public health problem. This project combines the Rice group's expertise in the structural biology and biochemistry of DNA recombinases with that of Drs. Daum and Boyle-Vavra, who are experts in SCCmec architecture as well as the molecular epidemiology, antibiotic resistance mechanisms, and virulence of MRSA. The specific aims of this R21 are to: 1) Develop an in vitro system for studying SCCmec-encoded DNA recombinases with purified components. and 2) Develop in vivo systems for studying these proteins in S. aureus and E. coli stains. The long-term goals are to understand the mechanism and regulation of these recombinases at the atomic level, to leverage this knowledge to understand the horizontal transmission of SCCmec, and to devise ways to stop its spread into new strains and/or trigger its excision and loss from the MRSA genome. PUBLIC HEALTH RELEVANCE: This work will lay the foundation for a long-term project to understand, at a molecular level, the mobility of the SCCmec element that renders Staphylococcus aureus resistant to a broad variety of penicillins. Insertion of the SCCmec DNA element into the S. aureus chromosome results in "MRSA" (methicillin resistant S. aureus) strains that have become a serious public health problem. Our study of the proteins that catalyze this DNA recombination reaction will help us understand the transmission of SCCmec between Staphylococcus species, and will lay the groundwork for devising ways to stop its spread into new strains and/or trigger its excision and loss from the MRSA genome.